More fixes for builddir != srcdir.

More fixes for builddir != srcdir.

 - Use absolute directories for $builddir and $srcdir in the Makefile

 - Don't try and combine source files in $builddir and $srcdir to build
   proto.h.  It's just too hard to get it right across all targets we
   wish to compile on.  Use a hand created prototype for the single
   function in smbd/build_options.c that we need.  This allows us to ditch
   all the extra sed work that was causing problems: \t not portable - hah!

 - Fix bogus delheaders target to remove the correct files

This appears to work quite nicely now.  Let's see how it goes on the
buildfarm machines.
(This used to be commit 456184463d35c18840c39cb3483b7136247ea764)